This invention relates to a passive steering assembly for a guided vehicle movable along a guideway and to a passive steering system using a plurality of such assemblies.
Rubber tired transit vehicles are commonly used in personal rapid transit guided vehicle systems. Most employ some type of steering system. Vehicle steering systems are used to generate lateral loads, typically at the tire patch, to turn vehicles and offset lateral vehicle inertia and wind loads. Such lateral loads can be as high as one quarter of the weight of the vehicle. There are three different types of tire steering system categories currently used as follows: (1) mechanical sensing with power steering; (2) electromagnetic (wire follower) sensing with power steering; and (3) mechanical sensing, passive steering.
The first two steering systems categories are known as active steering. The first typically employs a mechanical sensor which establishes the lateral distance between the vehicle and reference surface on the guideway such as the guideway""s vertical wall. This distance is used by an electronic controller which activates a power steering system, steers the wheels, and thereby generates the required steering forces which actively turn the vehicle. The forces generated by the tires can be as high as one quarter of the weight of the vehicle. The second type also uses an automatic controller to activate a power steering system, steer the wheels, and thereby generate the required steering forces which turn the vehicle. But, unlike the first system, the second system uses a transmission wire laid within the guideway as the vehicle position reference. Both of the first two steering systems suffer from certain deficiencies. First, since the vehicles are steered from lateral forces generated by the tires, vehicle roll stability when negotiating turns is adversely affected. To maximize vehicle roll stability it would be more advantageous to react lateral steering loads higher and closer to the vehicle center of gravity. To offset this disadvantage, such actively steered vehicles must employ a wider track width and hence a wider guideway. The second disadvantage is that both active steering systems are complex and costly. Guideways are wider and therefore heavier; wire following guideways are more complex, and both steering systems require elaborate controls and hydraulic/pneumatic steering drives.
The third steering system category is mechanical sensing with passive steering. Unlike the first two categories, there is no electronic controller or power steering. The vehicle follows a vertical surface of the guideway. Forces required to steer the vehicle wheels are developed through mechanical steering linkages which reach directly off of the guideway vertical surface. Prior art passive steering systems occur in two different configurations: wagon wheel and Ackermann. For wagon wheel passive steering, each axle is fixed to a rotatable truck. The tires always remain coaxial with the axle. The axis of truck rotation is vertical and located at the center of the axle (which is also the center of the truck). Each truck also has four lateral wheels (two per side). Two are located forward of the axle and two to the rear. The lateral wheels contact the sides of the guideway and thereby rotate the truck and steer the axle and wheels. Typically, there are two truck/axle assemblies per vehicle. Forward and reverse travel are allowable.
Passive Ackermann steering more closely resembles that found on an automotive vehicle. Here the wheels on each axle steer about kingpin axes located in close proximity to each wheel. Left and right wheels on each axle are tied together with a tie rod or relay link. The steering linkage is configured such that the inboard wheel turns slightly more than the outboard tire when in a turn (Ackermann steering). In prior art, a spring connected to the steering linkage biases the vehicle to steer into one of the guideway side walls. A mechanical link with cam follower attached to the steering linkage follows the side of the guideway. If the vehicle is too close to the side of the guideway, the follower steers the vehicle away from the side of the guideway. Steering equilibrium is reached when the tire lateral forces are offset by the cam follower""s lateral load on the guideway.
While the passively steered wagon wheel concept has the advantage of reverse travel capability, it has a number of drawbacks. First, is has more moving parts and would therefore tend to be less reliable. Second, the trucks are heavy. This has an adverse effect on guideway size, weight and cost. In addition, a heavier unsprung mass (truck) would have a negative impact on ride quality. Third, like the active steering systems, lateral vehicle forces are reacted well below the vehicle center of gravity, thereby leading to vehicle roll instability unless a wider and more massive guideway is employed. And fourth, experience shows that friction between each truck and the chassis frame is ample enough to prevent proper wheel/guideway alignment thereby leading to more tire wear. The passively steered Ackermann steering concept has certain advantages over the wagon wheel concept. They include smaller spatial requirements and lighter unsprung mass. This enables a compact design, less weight, and enhanced ride quality. Unfortunately prior art passive Ackermann also comes with some drawbacks. First, the tires are always scrubbing into one of the two guideway walls. This causes excessive tire wear and adversely affects ride quality. Second, like all prior art discussed above, steering forces are generated well below the vehicle center of gravity. This degrades vehicle roll stability when negotiating turns. Third, reverse travel is unstable.
It is therefore an object of this invention to provide an improved passive steering assembly for a guided vehicle and to an improved passive steering system employing a plurality of such passive steering assemblies.
It is a further object of this invention to provide such an improved passive steering assembly which reduces guideway size, mass and cost.
It is a further object of this invention to provide such an improved passive steering assembly which reduces lateral loads at the tires.
It is a further object of this invention to provide such an improved passive steering assembly which reduces tire wear and improves tractive capability.
It is a further object of this invention to provide such an improved passive steering assembly which reduces control and hydraulic/pneumatic equipment and complexity.
It is a further object of this invention to provide such an improved passive steering assembly which improves reliability.
It is a further object of this invention to provide such an improved passive steering assembly which accommodates reverse travel.
It is a further object of this invention to provide such an improved passive steering assembly which reduces lateral loads on the guideway.
It is a further object of this invention to provide such an improved passive steering assembly which has higher roll stability with narrower guideways.
It is a further object of this invention to provide such an improved passive steering assembly which can be implemented with essentially stock components.
It is a further object of this invention to provide such an improved passive steering assembly which steers passively by forces at the tire patch.
It is a further object of this invention to provide such an improved passive steering assembly which has small scrub radius to reduce upsetting steering moments during braking.
It is a further object of this invention to provide such an improved passive steering assembly which has a large caster angle to reduce lateral tire forces.
The invention results from the realization that a truly simple, safe, reliable and economical passive steering assembly for a guided vehicle which reduces the lateral forces on the tires can be achieved by using a lateral guide device such as the lateral suspension system or vehicle switching system of the vehicle to orient the vehicle in the guideway and permit the wheels to follow the path of the guideway and automatically, passively steer along it. Two further realizations are that: lateral forces at the tires can be reduced, stability and tire wear improved, and narrow guideways are possible by reacting vehicle lateral forces at or near the vehicle center of gravity and allowing the vertical support tires to passively steer; and that more accurate steering and load compensation from left to right tires is possible by using Ackerman type steering.
This invention features a passive steering assembly for a guided vehicle moveable along a guideway including a vehicle having at least one axle structure, a pivot joint at each end of the axle stricture, and a wheel mounting pivotably engaged with each pivot joint. There is a hub for rotatably mounting a wheel to each wheel mounting, a wheel connected to each hub for moving the vehicle, and a lateral guide device for controlling the lateral position of the axle structure for steering the wheels to follow the path of the guideway.
In a preferred embodiment there may be a tie rod interconnected between the wheel mountings for balancing steering loads on the wheels. There may be a locking device for fixing the wheel mountings to prevent steering the wheels when the vehicle moves in reverse. The locking device may include a locking unit interconnected with each wheel mounting. The lateral guide device may include a lateral suspension system and the lateral suspension system may interact with the guideway at approximately the height of the center of gravity of the vehicle. The lateral guide device may also include a vehicle switch system. The tie rod and the wheel mountings may be connected in an Ackermann steering linkage.
This invention also features a passive steering system for a guided vehicle movable along a guideway having a vehicle including two axle structures, a pivot joint at each end of each axle structure, and a wheel mounting pivotably engaged with each pivot joint. There is a hub for rotatably mounting a wheel to each wheel mounting and a wheel on each wheel mounting for movably supporting the vehicle. A lateral guide device controls the lateral position of the axle structures for steering the wheels to follow the path of the guideway.
In a preferred embodiment there may be a tie rod interconnected between the wheel mountings on each axle structure for balancing steering loads on the wheels. The tie rods may be interconnected with their respective wheels on the inboard side of the axle structures. The tie rod and wheel mountings may be connected in an Ackermann steering linkage.